Identification: White spots on my lemon tree - whats wrong?

Hello. I'm new to the forum. Writing from Sweden.

My 15 year old lemon tree that has always been in the same room and at the same window, got for the first time light yellow spots on its leaves 2 weeks ago as you can see on the attachment. My first reaction was lack of nourishment cuz I've been showering the tree a lot, so I put in some little extra, alltho its always getting nourishment all year around every week.

The leaves doesnt drop and overall, the tree looks healthy. It has a fruit growing, and some flowers are blooming here and there. Which is normal for me tree.

I started using a new label of nourishment from what I have been using before.

This year, in late winter, the tree got bugs for the first time in its life. I managed to kill the bugs off with a home made soap mix with alcohole just 2 moths ago - after the treatment, some of the leaves got a bit deformed. The tree, as I wrote above, has been in the shower a lot latley, maybe 6 times during a period of 2 months.

I was wondering if anybody know why the white spots appear, and if possible, be as specific as possible to what can be done to fix the problem of mine.